Circuit arrangement for generating square pulses and improved compensation current sensor using same

ABSTRACT

The circuit arrangement for generating square pulses according to a magnetic field strength has an edge-triggered flip-flop a comparator connected to the flip-flop; a bridge with four bridge segments, each including an electronically controlled switch, and with a transverse branch including an energy-storing element consisting of an inductive resistor acting as a magnetic field probe; and a switching threshold resistor connected with the energy-storing element and with a signal input of the comparator. The switches are each connected in pairs in crossover fashion by the flip-flop, so that current flow in the transverse branch is reversible. An improved compensation current sensor, which is less sensitive to component tolerances, includes the circuit arrangement for generating square pulses in a controller.

PRIOR ART

The invention relates to a circuit arrangement for generating squarepulses, having an edge-triggered flip-flop and at least one comparator,whose output is connected to the trigger input of the flip-flop, and anenergy-storing element, which is charged in alternation as a function ofthe switching state of the flip-flop, and at least one switchingthreshold resistor is connected in series with the energy-storingelement, at which resistor a voltage generated by the current flowingthrough the energy-storing element drops, which voltage is fed to thesignal input of the comparator.

Using such a circuit arrangement to generate square pulses is known. Theknown circuit is used for instance to measure the field intensity of amagnetic field. A magnetic field probe, which is embodied as aninductive resistor and represents the energy-storing element, is placedin the magnetic field to be measured. The magnetic field probe isembodied such that it is brought to saturation by the magnetic field tobe measured and the magnetic field generated by the current. As long asno external magnetic field acts on the magnetic field probe, or in otherwords the magnetic field to be measured is zero, the magnetic fieldprobe has an electrical behavior that, in terms of an electric currentflowing through it, is independent of the direction of the current. Thesquare pulses generated by the circuit arrangement, as a result, have apulse-duty factor of 1:1.

If an external magnetic field acts on the magnetic field probe, that is,if the magnetic field to be measured is no longer zero, then magneticfield probe reaches saturation earlier in one direction than in theother. With respect to an electric current flowing through it, itsbehavior is therefore no longer dependent on the direction of theelectric current. As a result, the pulse-duty factor of the squarepulses changes. Hence the pulse-duty factor of the square pulsesrepresents a measure of the magnetic field acting on the magnetic fieldprobe.

A known circuit arrangement is shown in FIG. 3. In the known circuitarrangement, two comparators are provided, whose outputs are fed to anAND gate 22, whose output is connected to the trigger input of theflip-flop 21. The signal inputs of the comparators are each connected toa different end of the energy store, that is, of the magnetic fieldprobe. The energy store is connected between the two outputs of theflip-flop. Thus depending on the position of the flip-flop, currentflows in a different direction through the energy store. Between theoutputs of the flip-flop and the energy store, respective switchingthreshold resistors are connected. The junction points of the switchingthreshold resistors and the energy store are each connected to thesignal input of a comparator. The two reference inputs of thecomparators are connected to one another, so that the same referencevoltage is present at both comparators.

At the output of the flip-flop at which there was no output voltage,there is now an output voltage after a switchover of the flip-flop, andthere is no longer an output voltage at the other output. By means ofthe energy stored in the coil, the original current flow is, however,maintained. As a result, the potential at the signal input of theapplicable comparator drops below the switching threshold. As aconsequence, the voltage at the output of the applicable comparatorbecomes zero. As a result, the output of the AND gate also becomes zero,so that the voltage at the signal input of the applicable comparatoragain reaches the switching threshold, causing the comparator again tooutput an output signal, and the AND gate is switched through. By meansof the edge occurring upon switching of the AND gate, the flip-flop istriggered again, so that it switches over once again, and the processjust described is repeated. The circuit is dimensioned such that itoscillates at a frequency of approximately 350 kHz.

However, the known circuit has the disadvantage that the tolerances ofthe switching threshold resistors and the tolerances of the switchingthresholds of the comparators affect the pulse-duty factor. Moreover,different delay times of the comparators adversely affect the symmetryof the circuit arrangement. The transit time of the AND gate alsoadversely affects the resolution achieved with the circuit. Furthermore,additional costs result from the use of an AND gate.

It is an object of the invention to provide a circuit arrangement forgenerating square pulses of the above-described kind in which thinfluence of tolerance in the components is lessened.

It is another object of the present invention to provide an improvedcompensation current sensor for current flowing in an electricallyconducting element with a controller, which includes the circuitarrangement for generating square pulses according to the invention.

ADVANTAGES OF THE INVENTION

According to the invention, a circuit arrangement for generating squarepulses, having an edge-triggered flip-flop and at least one comparator,whose output is connected to the trigger input of the flip-flop, and anenergy-storing element, which is charged in alternation as a function ofthe switching state of the flip-flop, and at least one switchingthreshold resistor is connected in series with the energy-storingelement, at which resistor a voltage generated by the current flowingthrough the energy-storing element drops, which voltage is fed to thesignal input of the comparator, is characterized in that theenergy-storing element is disposed in the transverse branch of a bridge,in each of the four bridge segments of which a respective switch isdisposed, and the switches are each connected in pairs in crossoverfashion by the flip-flop, so that the current flow in the transversebranch is reversible, and that the bridge is connected in series withthe switching threshold resistor, and the junction point of the bridgeto the switching threshold resistor is connected to the signal input ofthe comparator.

By means of the arrangement according to the invention, it isadvantageously unnecessary to have two switching threshold resistors.Not only is the space required reduced, but by using only one switchingthreshold resistor, asymmetries are avoided. This is because the currentflowing through the energy store always flows through the same switchingthreshold resistor, and thus the tolerance of the switching thresholdresistor has the same effect in both switching states of the flip-flop.

The same is true for a tolerance of the comparator. Since only onecomparator is used, tolerances in terms of the switching threshold andthe transit time in both switching states of the flip-flop have the sameeffect. Moreover, both space and expense are saved by using only onecomparator.

Since in the circuit arrangement of the invention an AND gate is nolonger necessary, the transit times caused by such an element cannothave any effect. Omitting the AND gate advantageously also saves space.

An embodiment of the invention in which the energy-storing element is aninductive resistor has proved especially advantageous. Because theenergy-storing element is an inductive resistor, it can be embodied as amagnetic field probe, as provided in a further particular embodiment ofthe invention. When the inductive resistor is embodied as a magneticfield probe, the circuit arrangement of the invention can especiallyadvantageously be used for measuring a magnetic field.

It is especially advantageous if the magnetic field probe is used todetect the magnetic field of a core of a compensation current sensor, asis contemplated in a further particular embodiment of the invention. Byusing the circuit arrangement of the invention in a compensation currentsensor, the accuracy of the compensation current sensor can be improvedin a simple way.

In an advantageous feature of the invention, the comparator is embodiedas a digital gate. As a result, the circuit can be produced quiteeconomically. An AND gate could for instance be used as the digitalgate. However, it has also proved advantageous to use an analogcomparator, which as its output signals furnishes digital signals thatare simple to process further.

In a further particular embodiment of the invention, it isadvantageously provided that the switches are MOSFETs, of which two aretriggered directly and two are triggered via inverters from the outputsof the flip-flop. Embodying the switches as MOSFETs makes it simple torealize the circuit arrangement of the invention. Moreover, triggeringof the switches is made simpler. In addition, the switches can betriggered quite precisely.

It has furthermore been found that it is especially advantageous if inthe transverse branch of the bridge, a series resistor is connected inseries with the energy-storing element. By means of the series resistor,the sensitivity of the circuit arrangement can be adjusted in a simpleway.

Further details, characteristics and advantages of the present inventionwill become apparent from the ensuing description of a particularexemplary embodiment in conjunction with the drawing.

DRAWING

Shown are:

FIG. 1, one embodiment of the circuit arrangement of the invention in aschematic diagram;

FIG. 2, a compensation current sensor in a schematic diagram; and

FIG. 3, a circuit arrangement of the prior art in a schematic diagram.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

As can be seen from FIG. 1, a coil 3 and a series resistor 11 connectedin series with it are disposed in the transverse branch of a bridge. Oneswitch 7, 8, 9, 10 embodied as a MOSFET is disposed in each of the fourbridge segments of the bridge. The first switch 7, disposed in one partof the bridge, and the second switch 8 are embodied as p-channel MOSFETsand connected to the operating voltage VCC. The third switch 9 disposedin the other part of the bridge and the fourth switch 10 are embodied asn-channel MOSFETs and connected to a switching threshold resistor 4. Theother end of the switching threshold resistor 4 is connected to ground.

The junction point between the n-channel MOSFETs 9, 10 and the switchingthreshold resistor 4 is connected to the signal input 2 a of acomparator 2. A reference voltage U_(ref) is applied to the referenceinput 2 b of the comparator 2. The output of the comparator 2 isconnected to the trigger input of an edge-triggered flip-flop 1. Thenon-inverting output la of the flip-flop 1 is connected, via a firstinverter 5, to the gate of the first, p-channel MOSFET 7 as well asbeing connected directly to the gate of the fourth, n-channel MOSFET 10,disposed in crossover fashion in the bridge. The inverting output 1 b ofthe flip-flop 1 is connected, via a second inverter 6, to the gate ofthe second, p-channel MOSFET 8 as well as being connected directly tothe gate of the third, n-channel MOSFET 9, disposed in crossover fashionin the bridge. The MOSFETs 7, 8, 9, 10 are thus connected through inpairs in crossover fashion (7 and 10, on the one hand, and 8 and 9, onthe other) by the flip-flop 1.

If the output voltage is applied to the non-inverting output la of theflip-flop 1, then the first MOSFET 7 and the fourth MOSFET 10 areswitched through. A current can thus flow from left to right in thetransverse branch of the bridge, or in other words can flow through theseries resistor 11 and the inductive resistor 3.

If the flip-flop 1 switches over, then the output voltage is applied tothe inverting output 1 b of the flip-flop 1. As a result, the firstMOSFET 7 and the fourth MOSFET 10 block, while conversely the secondMOSFET 8 and the third MOSFET 9 are switched through.

Because the current flow in FIG. 2 from left to right through thetransverse branch of the bridge is maintained as a result of the storageaction of the inductive resistor 3, the voltage at the signal input 2 aof the inverter 2 drops below the value of the reference voltage U_(ref)located at the reference input 2 b of the comparator 2. As a result, theoutput signal of the comparator becomes zero.

Once the direction of the current in the transverse branch of the bridgehas reversed, because of the switching through of the second MOSFET 8and the third MOSFET 9, or in other words once the current is nowflowing from right to left through the transverse branch of the bridgeor in other words through the inductive resistor 3 and the seriesresistor 11, the voltage drop brought about at the switching thresholdresistor 4 increases again. Once it has reached the switching thresholdof the comparator 2, the output voltage again prevails at the output ofthe comparator 2. By means of the edge created by the switchover of thecomparator 2, the flip-flop 1 is triggered, so that it switches overonce again. The switching cycle described above thus begins all overagain.

The comparator 2 is for instance an analog comparator, which at itsoutput furnishes a digital signal, which be processed simply in thedownstream digital circuit. As the comparator 2, it is for instance alsopossible to use a component designed as a digital gate, optionally witha suitably adapted circuit design.

In the circuit arrangement of a compensation current sensor, shown inFIG. 2, a field probe 12 is disposed in the air gap of an annular core13 that surrounds a conductor 15 whose current is to be measured. Thefield probe 12 is embodied as an inductive resistor, which reachessaturation as a result of the magnetic field in the air gap of theannular core 13. The field probe 12 is the inductive resistor 3 includedin FIG. 1, and it is disposed in a circuit arrangement of FIG. 1 thatforms part of a controller 16. The pulse-duty factor, brought about bythe magnetic field in the air gap of the annular core 13, of the squarepulses generated by the circuit arrangement in FIG. 1 is evaluated inthe controller 16 and used to generate a compensation current I_(A) Thecompensation current I_(A) is conducted through a coil 14 that is woundaround the annular core 13.

The controller 16 is designed such that the current flowing through thecoil 14 is high enough that the magnetic field in the air gap of theannular core 13 is nearly zero. As a result, the current I_(A) flowingthrough the coil 14 can be used as a measure for the current flowingthrough the conductor 15. To generate an output voltage U_(A), aresistor 17, at which the output voltage U_(A) drops, is connected inseries with the coil 14.

In FIG. 3, the known circuit arrangement, discussed in the backgroundsection above, is shown. As can be seen from FIG. 3, an inductiveresistor 25 is connected between the outputs of an edge-triggeredflip-flop 21. Depending on the position of the flip-flop 21, currentthus flows in a different direction through the inductive resistor 25. Arespective switching threshold resistor 26, 27 is connected between theoutputs of the flip-flop 21 and the inductive resistor 25. The junctionpoint of the switching threshold resistors 26, 27 is connected to thesignal input of a respective comparator 23, 24. The two reference inputsof the comparators 23, 24 are connected to one another, so that the samereference voltage is present at both comparators. The outputs of thecomparators 23, 24 are connected to the two inputs of an AND gate 22.The output signals of these comparators thus affect the switching statesof the AND gate. The output of the AND gate 22 is connected to thetrigger input of the flip-flop 21.

For further information on the mode of operation of the known circuitshown in FIG. 3, see the background section above.

What is claimed is:
 1. A circuit arrangement for generating squarepulses as a function of field intensity of a magnetic field, saidcircuit arrangement comprising an edge-triggered flip-flop (1) having atrigger input; at least one comparator (2) having a signal input (2 a)and an output connected to the trigger input of the flip-flop (1); abridge comprising four bridge segments connected in series with eachother and a transverse branch connected across said bridge between saidbridge segments, wherein respective bridge segments comprisecorresponding switches (7, 8, 9, 10), said transverse branch comprisesan energy-storing element (3) alternately charged as a function ofswitching state of the edge-triggered flip-flop (1) and saidenergy-storing element (3) consists of a magnetic field probe (12) fordetecting a magnetic field, which consists of an inductive resistor; andat least one switching threshold resistor (4) connected in series withsaid energy-storing element (3), which is connected with said signalinput (2 a) of said at least one comparator (2), so that a voltage isapplied to said signal input due to current flowing through theenergy-storing element (3); and wherein pairs (7, 10 or 8; 9) of saidswitches on opposite sides of the transverse branch are connected bysaid flip-flop (1) in crossover fashion, so that said current flowing insaid energy-storing element (3) is reversible, and wherein a junctionpoint of the bridge to the at least one switching threshold resistor (4)is connected to said signal input (2 a) of the at least one comparator(2).
 2. The circuit arrangement as defined in claim 1, wherein saidmagnetic fiord is generated by a core (13) of a compensation currentsensor.
 3. The circuit arrangement as defined in claim 1, wherein saidat least one comparator (2) is a digital gate.
 4. The circuitarrangement as defined in 17, further comprising inverters (5,6)connected to outputs of said flip-flop (1) and wherein said respectiveswitches (7, 8, 9.10) are MOSFETs, one of said outputs of said flip-flop(1) is directly connected to a first (10) of said switches andindirectly to a second (7) of said switches via an inverter (5) andanother of said outputs of said flip-flop (1) is directly connected to athird (9) of said switches and indirectly connected to a fourth (8) ofsaid switches via another inverter (6), in order to switch said switchesin said crossover fashion.
 5. The circuit arrangement as defined in 1,wherein said transverse branch comprises a series resistor (11)connected in series with said energy-storing element (3) or saidinductive resistor.
 6. A compensation current sensor for measuringcurrent flowing in an electrically conducting element, said compensationcurrent sensor comprising an annular core (13) with an air gap, throughwhich an electrical conductor carrying a current to be measured passes;an electrically conductive coli (14) wound around said annular core(13); a magnetic field probe (12) arranged in said air gap of saidannular core (13), said magnetic field probe consisting of an inductiveresistor and comprising means for detecting a magnetic field in said airgap of said annular core; and a controller (16) for generating acompensation current (I_(A)) passing through said coil (14); whereinsaid controller (16) comprises a circuit arrangement for generatingsquare pulses according to field intensity of said magnetic field ofsaid annular core, said circuit arrangement comprising an edge-triggeredflip-flop (1) having a trigger input; at least one comparator (2) havingan output connected to the trigger input of the flip-flop (1); a bridgecomprising four bridge segments connected in series with each other anda transverse branch connected across said bridge between said bridgesegments, wherein respective bridge segments comprise correspondingswitches (7, 8, 9, 10), said transverse branch comprises annergy-storing element (3) alternately charged as a function of switchingstate of the flip-flop (1) and said energy-storing element (3) consistsof said magnetic field probe (12), which consists of an inductiveresistor; and at least one switching threshold resistor (4) connected inseries with said energy-storing element (3), which is connected with asignal input (2 a) of said at least one comparator (2), so that avoltage is applied to said signal input due to current flowing throughthe energy-storing element (3); wherein said switches are connected inpairs in crossover fashion by said flip-flop (1), so that said currentflowing in said energy-storing element (3) is reversible, and wherein ajunction point of the bridge to the at least one switching thresholdresistor (4) is connected to said signal input (2 a) of the at least onecomparator (2).
 7. The compensation current sensor as defined in claim6, wherein said controller (16) Includes means for generating acompensation current (I_(A)) from a pulse-duty factor of said squarepulses and means for conducting said compensation current (I_(A))through said coil (14) wound around said annular core (13), whereby saidfield intensity of said magnetic field is nearly zero.
 8. Thecompensation current sensor as defined in claim 6, wherein said at leastone comparator (2) is a digital gate.
 9. The compensation current sensoras defined in claim 6, wherein said circuit arrangement includesinverters (5,6) connected to outputs of said flip-flop (1) and whereinsaid respective switches (7, 8, 9, 10) are MOSFETs, one of said outputsof said flip-flop (1) Is directly connected to a first (10) of saidswitches and indirectly to a second (7) of said switches via an inverter(5) and another of said outputs of said flip-flop (1) is directlyconnected to a third (9) of said switches and indirectly connected to afourth (8) of said switches via another inverter (6) connected toanother of said outputs of said flip-flop (1), in order to switch saidswitches in said crossover fashion.
 10. The compensation current sensoras defined in claim 6, wherein said transverse branch comprises a seriesresistor (11) connected in series with said energy-storing element orsaid inductive resistor.